1. Field of the Invention
This invention relates to improvements in Fourier transform ion cyclotron resonance mass spectrometers.
2. Brief Description of the Prior Art
Mass spectrometry is an analytical technique for identification of chemical structures, determination of mixtures and quantitative elemental analysis based upon application of the mass spectrometer. Fourier transform ion cyclotron mass spectrometers generally include an evacuated chamber into which is leaked a gaseous form of a neutral species to be analyzed while the pressure in the chamber is maintained at about 1.times.10.sup.-5 mm Hg. A gated electron beam is sent the length of the vacuum chamber through the center of the chamber. Electrons collide with the molecules of gas present and remove an electron from the molecules, thus creating a collection of low energy (thermal) positive ions which precess in a circle since they are disposed in a magnetic field directed along the chamber axis.
The ions are excited by applying a radio frequency (RF) "sweep" pulse which sweeps from a frequency below the lowest resonance frequency of the species involved to a frequency above the highest resonance frequency of the species involved, generally from about 200 KHz to about 2 MHz, to a pair of parallel excite electrodes in the chamber to provide an electric field resulting from the application of a potential difference of from a fraction of a volt to a few volts, depending upon the cell geometry, ion population and other factors, across the excite electrodes. As the pulse sweeps through the frequency range which represents the fundamental cyclotron frequencies of all ions of interest, this energy is absorbed by the ions when their fundamental oscillating frequency has been reached. This causes the ions to "spin up" to larger orbitals in order that the ions move closer to a pair of detect electrodes within the chamber which are generally parallel to each other and generally normal to the excite electrodes. As the ions are spinning up into these higher orbits, they fall into coherent packets, that is, they tend to draw together as they absorb the excite energy, thereby creating an iso-mass bundle of identical ions. This packet then resembles a relatively large, highly charged particle which orbits the chamber at the fundamental cyclotron frequency of the ions which make up this packet.
Ion packets are detected by monitoring the current induced by the ion packets onto the passive detect electrodes, this appearing as a sine wave. The frequency of the sine wave is then determined and, from known laws of physics, provides the mass of the ion under test. When plural ions are present, there is a superposition of several sine waves, requiring that a Fourier transform be performed thereon to obtain an indication of each of the distinct frequencies present. The above described procedure can be repeated many times per second to permit signal averaging and averaging out of noise, thereby providing improved signal/noise ratios.
Fourier transform ion cyclotron resonance mass spectrometers (FTICR/MS) have been known for some time. FTICR/MSs provide the same information as standard mass spectrometers. However, they are very precise in that they provide data which are orders of magnitude more precise than ordinary mass spectrometers and orders of magnitude more sensitive (possibly detecting as little as one ion) than ordinary mass spectrometers. Since the identification of the species under test is determined from the mass or atomic weight of the species under test, it is imperative that the instrumentation provide accuracy to a sufficient number of decimal places to differentiate between two species having very close atomic weights. For example, nitrogen and carbon monoxide both have an atomic weight of 28 and cannot be differentiated from each other when results can only be accurately provided to one decimal place. This is also true for nitrogen and carbon monoxide until accuracy to three decimal places can be obtained. Only at this point is there a difference in the atomic weights of these species wherein the atomic weights of the species are unique.
The use of such instruments has been limited in the past due to their enormous cost and large size. The prior art FTICR/MSs have required a superconducting magnet which requires liquid nitrogen and liquid helium for operation. The physical dimensions of the prior art FTICR/MSs have approached the dimensions of a small room whereas the strength of the magnet required in such instruments is such that it requires a roped off or off limits area therearound which is equal to about four times the area required for the FTICR/MS itself. It follows that FTICR/MSs of the prior art are not portable and therefore also find restricted use for that reason as well. It is therefore apparent that a FTICR/MS which can perform the function of prior art FTICR/MSs, yet be capable of fabrication at reduced cost and/or occupy a reduced amount of space and/or be portable would be a highly desirable instrument.